U.S. patent application number 09/940493 was filed with the patent office on 2002-03-07 for image forming apparatus.
Invention is credited to Maebashi, Yoichiro, Nakai, Tomoaki.
Application Number | 20020028084 09/940493 |
Document ID | / |
Family ID | 18749464 |
Filed Date | 2002-03-07 |
United States Patent
Application |
20020028084 |
Kind Code |
A1 |
Maebashi, Yoichiro ; et
al. |
March 7, 2002 |
Image forming apparatus
Abstract
The invention provides an image forming apparatus and a method
of manufacturing thereof which is capable of continuously forming
images with proper density without reducing the image forming speed
and, retrains toner consumption. A CPU for calculating an image
forming condition by a density sensor is provided. The image
forming operation is conducted in basis of a calculated new image
forming condition and a current image forming condition used prior
to the calculation of a new image forming condition in the image
forming operation so as to determine an available image forming
condition.
Inventors: |
Maebashi, Yoichiro;
(Shizuoka, JP) ; Nakai, Tomoaki; (Shizuoka,
JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
18749464 |
Appl. No.: |
09/940493 |
Filed: |
August 29, 2001 |
Current U.S.
Class: |
399/49 ;
399/55 |
Current CPC
Class: |
G03G 2215/0177 20130101;
G03G 2221/183 20130101; G03G 15/5041 20130101; G03G 2215/00042
20130101 |
Class at
Publication: |
399/49 ;
399/55 |
International
Class: |
G03G 015/00; G03G
015/06 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 30, 2000 |
JP |
261657/2000 |
Claims
What is claimed is:
1. An image forming apparatus comprising: an image forming means
for forming an image on a recording material; a detecting means for
detecting an image density; and a changing means for changing a
former image formation condition for the image forming means to a
next image formation condition for the image forming means on the
basis of a detection result of the detecting means, wherein the
changing means is capable of changing the former image formation
condition so as to bring it close to the next image formation
condition through a stepwise change of image formation
condition.
2. An image forming apparatus according to claim 1, wherein when a
first value corresponding to the next image formation condition is
smaller than a second value corresponding to the former image
formation condition, subtraction of a predetermined value is
performed on the second value to thereby determine the stepwise
change of image formation condition, and wherein when the first
value corresponding to the next image formation condition is larger
than the second value corresponding to the former image formation
condition, a predetermined value is added to the second value to
thereby determine the stepwise change of image formation
condition.
3. An image forming apparatus according to claim 1, wherein when a
first value corresponding to the next image formation condition is
smaller than a second value corresponding to the former image
formation condition, subtraction is performed in accordance with a
rate of change of the second value and the first value to thereby
determine the stepwise change of image formation condition, and
wherein when the first value corresponding to the next image
formation condition is larger than the second value corresponding
to the former image formation condition, addition is performed in
accordance with the rate of change to thereby determine the
stepwise change of image formation condition.
4. An image forming apparatus according to claim 1, wherein the
stepwise change of image formation condition is effected each time
image forming operation is performed by the image forming
means.
5. An image forming apparatus according to claim 2 or 3, wherein
when the subtraction results in a value not larger than the first
value, or when the addition results in a value not smaller than the
first value, an image formation condition corresponding to the
first value is used.
6. An image forming apparatus according to claim 1, wherein when a
difference between the former image formation condition and the
next image formation condition is within a predetermined range, the
former image formation condition is set without performing the
stepwise change of image formation condition.
7. An image forming apparatus according to claim 1, wherein when,
after a power source of the apparatus is turned on, the image
density is detected by the detecting means before the image
formation by the image forming means is started, the first image
formation condition of the image forming means is set without
performing the stepwise change of image formation condition.
8. An image forming apparatus according to claim 1, wherein when
the image density is detected by the image detecting means after a
predetermined period of time when the apparatus has not been used,
the first image formation condition of the image forming means is
set without performing the stepwise change of image formation
condition.
9. An image forming apparatus according to claim 1, wherein the
apparatus has a process cartridge detachable with respect to a main
body of the apparatus, wherein the process cartridge is equipped
with an image bearing member and a charging means for charging the
image bearing member, and wherein when the image density is
detected by the detecting means after the process cartridge is
replaced, the first image formation condition of the image forming
means is set without performing the stepwise change of image
formation condition.
10. An image forming apparatus according to claim 1, wherein the
image forming means includes an image bearing member, a toner image
forming means for forming a toner image on the image bearing
member, and a transfer means for transferring the toner image to
the recording material.
11. An image forming apparatus according to claim 10, wherein the
image formation condition is a toner image formation condition for
the toner image forming means.
12. An image forming apparatus according to claim 11, wherein the
toner image forming means is equipped with an electrostatic image
forming means for forming an electrostatic image on the image
bearing member and a developing means for developing the
electrostatic image with toner, and wherein the image formation
condition is at least one of an electrostatic image formation
condition for the electrostatic image forming means and a
development condition for the developing means.
13. An image forming apparatus according to claim 12, wherein the
development condition is a DC voltage applied to the developing
means.
14. An image forming apparatus according to claim 12, wherein the
image bearing member is a photosensitive member, wherein the
electrostatic image forming means is equipped with a charging means
for charging the photosensitive member and an exposure means for
subjecting the photosensitive member charged by the charging means
to exposure, and wherein the electrostatic image formation
condition is at least one of a charging condition for the charging
means and an exposure condition for the exposure means.
15. An image forming apparatus according to claim 1, wherein the
detecting means detects the image density each time the image
forming means has formed a predetermined number of images.
16. An image forming apparatus according to claim 1, wherein the
detecting means is a detection sensor for detecting the density of
a toner image.
17. An image forming apparatus according to claim 16, wherein the
detection sensor is equipped with a light emitting portion for
applying light to the toner image and a light receiving portion for
receiving light applied to the toner image.
18. An image forming apparatus according to claim 1, wherein the
image forming means is capable of forming a color image on the
recording material.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an image forming apparatus
for forming images on recording materials such as sheet
materials.
[0003] 2. Description of the Related Art
[0004] A conventional image forming apparatus will be described
with reference to FIG. 8, which is a sectional view of a
conventional color image forming apparatus.
[0005] Referring to the drawing, a photosensitive drum 1 serving as
an image bearing member is driven in the direction indicated by the
arrow by a driving means (not shown); it is charged uniformly by a
primary charger 2.
[0006] Then, a laser beam L which is in conformity with an yellow
image is applied to the photosensitive drum 1 from an exposure
device 3, whereby a latent image is formed on the photosensitive
drum 1.
[0007] As the photosensitive drum 1 further rotates in the
direction of the arrow, a developing device 4a, containing yellow
toner, of four developing devices 4a (yellow), 4b (magenta), 4c
(cyan), and 4d (black) supported by a rotation supporting means 11
rotates to come to be opposed to the photosensitive drum 1, and the
image is visualized by the yellow developing device 4a
selected.
[0008] An intermediate transfer belt 5 rotates in the direction of
the arrow substantially at the same speed as the photosensitive
drum 1. The toner image formed and borne on the photosensitive drum
1 under goes primary transfer to the outer surface of the
intermediate transfer belt 5 by a primary transfer bias applied to
a primary transfer roller 8a.
[0009] The above-described process is performed for the four
colors: yellow (hereinafter referred to as Y), magenta (hereinafter
referred to as M), cyan (hereinafter referred to as C), and black
(hereinafter referred to as K) whereby a toner image of a plurality
of colors is formed on the intermediate transfer belt 5.
[0010] Next, a transfer material is fed with a predetermined timing
from a transfer material cassette 12 by means of pick-up rollers
13.
[0011] At the same time, a secondary transfer bias is applied to a
secondary transfer roller 8b, and the toner image is transferred
from the intermediate transfer belt 5 to the transfer material.
[0012] Further, the transfer material is conveyed by a conveyance
belt 14 to a fixing device 6, where fusion and fixing are effected,
whereby a color image is obtained.
[0013] The toner remaining on the intermediate transfer belt 5 is
removed by an intermediate transfer belt cleaner 15.
[0014] On the other hand, the toner remaining on the photosensitive
drum 1 is removed by a cleaning device 7 consisting of a well-known
blade means.
[0015] When using the image forming apparatus described above,
maintenance operations, such as toner replenishment, the disposal
of waste toner, and the replacement of the photosensitive drum 1
when it has been worn.
[0016] In this example, the photosensitive drum 1, the primary
charger 2, and the cleaning device 7 are integrated into a process
cartridge A, and the developing devices 4a, 4b, 4c, and 4d are also
in the form of a developing cartridge which is easily detachable
with respect to the apparatus main body, so that the maintenance
operations can be easily conducted by the user.
[0017] Generally speaking, in an electrophotographic image forming
apparatus, fluctuations in the density characteristics of the
printed image are caused by the fluctuations in characteristics due
to the use environment, the developing device, the number of sheets
on which printing has been effected by the photosensitive drum, the
variation in sensitivity generated at the time of the production of
the photosensitive drum, the variation in frictional charging
characteristics generated at the time of the manufacturing of the
toner, etc.
[0018] Although strenuous efforts have been put into stabilizing
the characteristics in the variations and fluctuations, no
satisfactory result has been achieved yet.
[0019] In particular, in a color image forming apparatus, it is
necessary to adjust the conditions for the image formation in the
four colors of Y, M, C, and K before the user can achieve a desired
density and color balance.
[0020] In view of this, in the color image forming apparatus of
this example, a plurality of toner images for detection are formed
on the photosensitive drum 1 by varying the image forming condition
stepwise, and the reflection light quantity thereof is measured by
a density sensor 9. On the basis of the result of the measurement,
an image forming condition which is likely to provide a desired
density (reflection light quantity) is computed by a CPU 17 of the
main body for image density control.
[0021] Thus, the CPU 17 and the density sensor 9 correspond to
image formation condition computing means constituting elements of
the present invention used in the embodiment described below.
[0022] Next, the density sensor 9 will be described with reference
to FIG. 9, which is a schematic view of the density sensor applied
to the image forming apparatus shown in FIG. 8.
[0023] The density sensor 9 is composed of a light emitting element
91 such as LED, a photoreceptor 92 such as a photo diode, and a
holder 93. Infrared radiation from the light emitting element 91 is
applied to a patch P on the photosensitive drum, and the reflected
light therefrom is measured by the photoreceptor 92, whereby the
density of the patch P is measured.
[0024] The reflected light from the patch P contains a regular
reflection component and an irregular reflection component. The
light quantity of the regular reflection component undergoes great
fluctuations depending on the condition of the photosensitive drum
surface underneath the patch and fluctuation in the distance
between the sensor and the patch. Thus, when the reflected light
from the patch to be measured contains a regular reflection
component, the detection accuracy deteriorates to a marked
degree.
[0025] In view of this, in the density sensor 9, in order that no
regular reflection component from the patch P may impinge on the
photoreceptor 92, the angle at which light is applied to the patch
P is set to 45.degree. and the reception angle of the reflected
light from the patch P is set to 0.degree. with respect to the
normal I, thus measuring only the irregular reflection
component.
[0026] Next, the image density control in the color image forming
apparatus of this example will be described in detail.
[0027] First, the photosensitive drum 1 is charged by the primary
charger 2 such that its surface potential becomes -600V.
[0028] The sensitivity of the photosensitive drum and the exposure
amount of the laser are adjusted beforehand such that the potential
of the laser exposure portion at normal temperature and normal
humidity (23.degree. C., 60% Rh) is approximately -200V.
[0029] The developing bias is obtained by super imposing a
rectangular wave (with a frequency of 2000 Hz, 1800 Vpp) on a DC
voltage, as shown in FIG. 10. By making the DC voltage component
Vdc variable, the toner development amount is controlled. FIG. 10
is a graph depicting the developing bias applied to the image
forming apparatus shown in FIG. 8.
[0030] Further, prior to normal image formation, a plurality of
toner image patches of 30 mm square are printed at intervals, as
shown in FIG. 11, on the portion of the drum corresponding to the
density sensor 9. FIG. 11 is a schematic diagram showing patches
for density detection applied to the image forming apparatus shown
in FIG. 8.
[0031] The image patches are each developed by developing biases
with different DC voltage components, and reflection light quantity
measurement is performed on each of them by the density sensor 9.
In this example, the number of image patches is five, the DC
component Vdc of the developing bias being varied from -300V to
-500V in steps of 50V.
[0032] FIG. 12 shows an example of the result of reflection density
measurement. FIG. 12 is a graph showing the relationship between
reflection density and developing bias in the image forming
apparatus shown in FIG. 8.
[0033] In this example, the target value of the reflection density
of the toner (proper density value) is 1.4, and control is effected
such that image formation is conducted under a developing condition
estimated to be closest thereto (in this example, the DC voltage
component of the developing bias).
[0034] In this example, reflection density data as indicated by the
five points in FIG. 12 was obtained. The developing condition
providing the reflection density of 1.4 lies in the section where
the DC component Vdc is between -400V and -450V. Assuming that DC
component is approximately proportional to reflection density in
this section, it is to be assumed, through interior division of the
section between -400V and -450V, that the reflection density is 1.4
when the DC component is approximately -420V.
[0035] Thus, in this example, the DC component Vdc of the
developing bias as an image formation condition is controlled to be
-420V.
[0036] The above-described control is executed for each of the
colors, Y, M, C, and K, whereby the image density control is
completed.
[0037] The image density control is executed prior to image
formation (printing) each time printing is performed on a
predetermined number of sheets, when the power source of the main
body is ON, when replacing the process cartridge A or the
development cartridges (developing devices) 4a, 4b, 4c, and 4d, and
when a printing command is received when the apparatus has not been
in use for a long period of time.
[0038] While in this example the number of image patches is five,
it is also possible to increase the number to vary the developing
bias in more steps, thereby performing control more accurately.
[0039] When the variation in image density is too great to be coped
with solely by adjusting the developing bias, it is also possible
to perform control by combining other image formation conditions,
such as charging condition and exposure condition (exposure
amount)
[0040] The conventional color image forming apparatus, however, has
the following problems.
[0041] As described above, in an electrophotographic image forming
apparatus, the developing characteristics of the developing device
and the photosensitive characteristics of the photosensitive drum
fluctuate according to the condition of use of the apparatus, with
the result that the image density varies.
[0042] In particular, when printing is conducted successively, the
above-mentioned fluctuations in characteristics are more
conspicuous, and the image density is greatly varied.
[0043] Thus, each time printing is performed on a fixed number of
sheets, the above-described image density control is executed,
whereby the image density is prevented from being too much deviated
from the proper value.
[0044] However, even if such control is performed, fluctuation in
density naturally occurs between image density control
operations.
[0045] And, when the fluctuation in density occurs to a large
degree, a marked difference in density is generated before and
after the execution of the image density control.
[0046] This will be explained in detail with reference to FIG. 13,
which is a graph showing the variation in image density when
printing is successively executed in a conventional image forming
apparatus.
[0047] In the drawing, the vertical axis indicates density, and the
horizontal axis indicates the number of sheets on which printing is
performed. Broken line A indicates the proper image density for the
apparatus, and broken line B indicates how the image density will
change when image density control is not conducted each time
printing has been conducted on a fixed number of sheets.
[0048] At the left-hand end (X0) of the graph, image density
control is effected, and the image density is adjusted to the
proper density.
[0049] As can be seen from the graph, if density control is not
executed each time printing has been performed on a fixed number of
sheets, the image density will continue to increase to be greatly
deviated from the proper density.
[0050] Thus, it is necessary to conduct image density control each
time printing has been performed on a fixed number of sheets. Solid
line C indicates how the image density changes when image density
control is performed.
[0051] In this example, image density control is effected each time
printing has been performed on 100 sheets. Density control is
effected at points in time indicated by numerals X1 and X2 in the
drawing.
[0052] By thus performing image density control, the image density
is prevented from being greatly deviated from the proper density
for a long period of time.
[0053] However, there occurs a marked fluctuation in density
between the execution of image density control (indicated by X1 and
X2 in the drawing).
[0054] Suppose the user successively conducts the printing of the
same image before and after density control. For example, if
printing is successively performed on twenty sheets, for example,
from the 90th to 110th sheet, there is the possibility of the image
density of the first ten sheets being greatly different from that
of the ten sheets after density control.
[0055] In particular, in the case of a color image forming
apparatus like that of this example, in which a full color image is
reproduced by superimposing four color toner images one upon the
other, a great variation in the density of a particular color (one
of Y, M, C, and K) results in a marked change in the hue of the
image which is very conspicuous.
[0056] FIG. 14 shows an example in which, conversely to the case of
FIG. 13, image density is gradually reduced. As can be seen from
this graph, a similar problem is involved also in this case. FIG.
14 is a graph showing the variation in image density when printing
is successively executed in a conventional image forming
apparatus.
[0057] The above problem might be coped with by frequently
performing image density control. In that case, however, the
requisite time for density control would cause a reduction in the
printing speed. Moreover, that would involve the consumption of a
lot of toner for density control.
SUMMARY OF THE INVENTION
[0058] It is an object of the present invention to provide an image
forming apparatus which is capable of preventing an abrupt
fluctuation in image density.
[0059] Another object of the present invention is to provide an
image forming apparatus, in which the difference in image density
between an initial stage of use and a stage after long use is
reduced.
[0060] Still another object of the present invention is to provide
an image forming apparatus in which a reduction in image forming
speed when changing an image formation condition is mitigated.
[0061] A further object of the present invention is to provide an
image forming apparatus in which toner consumption for changing an
image formation condition is restrained.
[0062] A further object of the present invention is to provide an
image forming apparatus in which it is possible to change an image
formation condition gradually and stepwise.
[0063] These and still other objects, advantages and benefits may
be achieved using an image forming apparatus comprising:
[0064] an image forming means for forming an image on a recording
material;
[0065] a detecting means for detecting an image density; and
[0066] a changing means for changing a former image formation
condition for the image forming means to a next image formation
condition for the image forming means on the basis of a detection
result of the detecting means, wherein the changing means is
capable of changing the former image formation condition so as to
bring it close to the next image formation condition through a
stepwise change of image formation condition
[0067] Further objects and features of the present invention will
become more apparent from the following detailed description with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0068] FIG. 1 is a flowchart showing an image forming operation
applicable to a first embodiment of the image forming apparatus of
the present invention;
[0069] FIG. 2 is a flowchart showing an image forming operation
applicable to the first embodiment of the image forming apparatus
of the present invention;
[0070] FIGS. 3A and 3B are graphs showing how developing bias and
density change with the number of print sheets in the first
embodiment of the image forming apparatus of the present
invention;
[0071] FIG. 4 is a flowchart showing an image forming operation
applicable to a second embodiment of the image forming apparatus of
the present invention;
[0072] FIGS. 5A and 5B are graphs showing how developing bias and
density change with the number of print sheets in the second
embodiment of the image forming apparatus of the present
invention;
[0073] FIG. 6 is a flowchart showing an image forming operation
applicable to a third embodiment of the image forming apparatus of
the present invention;
[0074] FIGS. 7A and 7B are graphs showing how developing bias and
density change with the number of print sheets in the third
embodiment of the image forming apparatus of the present
invention;
[0075] FIG. 8 is a sectional view of a conventional color image
forming apparatus;
[0076] FIG. 9 is a schematic view of a density sensor applicable to
the image forming apparatus shown in FIG. 8;
[0077] FIG. 10 is a graph showing a developing bias applicable to
the image forming apparatus shown in FIG. 8;
[0078] FIG. 11 is a schematic view of density detection patches
applicable to the image forming apparatus shown in FIG. 8;
[0079] FIG. 12 is a graph showing the relationship between
reflection density and developing bias in the image forming
apparatus shown in FIG. 8;
[0080] FIG. 13 is a graph showing how image density changes when
printing is successively executed in a conventional image forming
apparatus; and
[0081] FIG. 14 is a graph showing how image density changes when
printing is successively executed in a conventional image forming
apparatus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0082] Preferred embodiments of this invention will now be
described in detail with reference to the drawings. It is to be
noted that the dimensions, materials, configurations, positional
relationships, etc. of the components described below should not be
construed restrictively unless otherwise specified.
[0083] Further, the components which are similar to those of the
prior art described above and those which are used in the
above-mentioned figures are indicated by the same reference
numerals. Further, it should be noted that the following
description of the embodiments of the image forming apparatus of
the present invention also serves as the description of the
embodiments of the image forming method of the present
invention.
First Embodiment
[0084] First, a first embodiment of the image forming apparatus of
the present invention will be described. In the image forming
apparatus of this embodiment, a gradual increase or decrease in
image formation condition is effected, from a first image formation
condition selected immediately before the execution of image
density control toward a second image formation condition
calculated through image density control, whereby an abrupt
variation in density is prevented.
[0085] The main construction of the color image forming apparatus
used in this embodiment is the same as that of the conventional
color image forming apparatus described with reference to FIG. 8,
so that a detailed description thereof will be omitted, and the
components shown in FIG. 8 will be referred to as appropriate.
[0086] In this embodiment, the DC component of a developing bias,
which constitutes a developing condition, is used as the image
formation condition to be changed so as to control image
density.
[0087] First, with reference to the flowchart of FIG. 1, the image
density control of this embodiment will be described in detail.
FIG. 1 is a flowchart showing an image forming operation applicable
to the first embodiment of the image forming apparatus of the
present invention.
[0088] First, when an execution command for image density control
is input to the CPU 17 of the main body, an image density control
sequence is started.
[0089] In this embodiment, image density control is executed in any
one of the following conditions.
[0090] 1. When the apparatus main body power source is ON (the
period between the turning ON of the power source and the
completion of preparation for image formation).
[0091] 2. When the process cartridge A or the developing cartridges
(developing devices) 4a, 4b, 4c, and 4d are replaced.
[0092] 3. When a printing command is received when the apparatus
has not been used for a long period of time (one hour in this
embodiment; this period can naturally be changed arbitrarily, which
also applies to the following embodiments).
[0093] 4. When printing has been performed on a predetermined
number of sheets (100 sheets in this embodiment; the number can
naturally be changed arbitrarily, which also applies to the
following embodiments).
[0094] Step 1
[0095] First, toner images for detection (toner patches) are formed
on the photosensitive drum 1. For each of the colors Y, M, C, and
K, five toner patches are formed, varying the DC component Vdc of
the developing bias from -300V to -500V in steps of 50V.
[0096] Step 2
[0097] The densities of the toner patches formed in Step 1 are
measured by the density sensor 9.
[0098] Step 3
[0099] From the results of the measurement of the toner patch
densities, the CPU 17, for example, calculates an optimum DC
voltage (optimum developing bias) 0.
[0100] Here, the value of the optimum developing bias 0 is a value
at which the toner patch density is 1.4, which is the proper
density for this image forming apparatus.
[0101] The optimum developing bias value 0 obtained is stored in a
memory (not shown) in the main body.
[0102] The main body memory may be volatile or nonvolatile. In this
embodiment, a volatile memory is used.
[0103] Step 4
[0104] A judgment is made as to whether image density control is to
be executed when the main body power source is ON. Immediately
after the turning ON of the power source, the voltage used by the
apparatus before the turning ON of the power source is unknown, so
that the value of the optimum developing bias 0 calculated
immediately after the control is used.
[0105] In the color image forming apparatus of this embodiment, a
print developing bias 1 is prepared as the developing bias value to
be used at the time of printing, separately from the optimum
developing bias value 0, and is stored in the main body memory.
[0106] Thus, when it is determined that the image density control
is that which is executed immediately after the turning ON of the
main body power source, the procedure advances to Step 7, and the
optimum developing bias 0 obtained through image density control is
input to the print developing bias value 1. Step 5 and Step 6
[0107] A judgment is made as to whether the image density control
is that which is executed immediately after the replacement of the
cartridge (the process cartridge A or the developing cartridge)
(Step 5).
[0108] Similarly, a judgment is made as to whether the image
density control is that which is executed when a printing command
is received when the apparatus has not been used for a long period
of time (which is one hour in this embodiment) (Step 6).
[0109] In either case, it is desirable to use the value of the
optimum developing bias value 0 calculated immediately after
density control, so that the procedure advances to Step 7, where
the optimum developing bias 0 obtained through image density
control is input to the print developing bias value 1.
[0110] When none of the conditions of Step 4, Step 5, and Step 6 is
applicable, the control is completed without updating the print
developing bias value 1.
[0111] In this case, the image density control executed is that
which is conducted when printing has been performed on a
predetermined number of sheets (100 sheets in this embodiment).
[0112] Thus, the print developing bias value 1 stored in the main
body memory is the developing bias value used immediately before
the image density control.
[0113] The above image density control is performed on each of the
colors Y, M, C, and K, and the image density control is
completed.
[0114] It goes without saying that the optimum developing bias
value 0 and the print developing bias value 1 are provided
independently for each of the colors (Y, M, C, and K), and stored
in the main body memory for the respective colors.
[0115] Next, the developing bias control at the time of printing
will be described with reference to the flowchart of FIG. 2. FIG. 2
is a flowchart illustrating an image forming operation applicable
to the first embodiment of the image forming apparatus of the
present invention.
[0116] The developing bias calculation at the time of printing is
conducted for each print sheet. That is, each time printing is
performed, the operation of the flowchart is started and executed.
Step 21
[0117] First, the developing bias value 1 used in the previous
printing is compared with the developing bias value 0 calculated
through image density control. When 0 is larger than 1, the
procedure advances to Step 22.
[0118] Step 22
[0119] In Step 22, a developing bias adjustment value is added to
the developing bias value 1 used in the previous printing (the
value corresponding to the image formation condition before
changing) to update the print developing bias value 1.
[0120] The developing bias adjustment value is an adjustment value
for adjusting and varying the developing bias for each print sheet;
it is preferably set to an optimum value according to the
characteristics of the apparatus.
[0121] Briefly, when this adjustment value is set to a small value,
the fluctuation in density for each print sheet is diminished.
When, conversely, it is set to a large value, the fluctuation in
density increases.
[0122] On the other hand, when the adjustment value is set to a
small value, the time it takes for the print developing bias 1 to
converge to the optimum developing bias 0 increases. When,
conversely, it is set to a large value, the requisite time for
convergence decreases.
[0123] Taking the above reason into consideration, the developing
bias adjustment value is set to 0.5V in this embodiment.
[0124] Step 23
[0125] The updated bias value 1 is compared with the optimum
developing bias 0 calculated through image density control (the
value corresponding to the image formation condition after the
change).
[0126] When 1 is not in excess of 0 yet, the procedure advances to
Step 29, where a developing bias output from a high voltage power
source is set to the value of 1.
[0127] When 1 has exceeded 0, the procedure advances to Step 24,
where the value of 1 is restored to 0 to effect updating. Step 25,
Step 26, Step 27, and Step 28
[0128] First, when the developing bias value 1 used in the previous
printing is larger than the optimum developing bias 0 calculated
through image density control, a computation reverse to that of
Step 21, Step 22, Step 23, and Step 24 is conducted to similarly
update the print developing bias value 1.
[0129] When none of the conditions of Step 21 and Step 25 is
satisfied, it means that the print developing bias 1 is equal to
the optimum developing bias 0, so that no updating of 1 is
effected.
[0130] Step 30
[0131] Using the print developing bias 1 updated through the above
computation, printing is performed.
[0132] It goes without saying that the print developing bias value
1 is calculated independently for each of the colors (Y, M, C, and
K).
[0133] Next, with reference to FIG. 3, the changes in the
developing bias and density in this embodiment will be described.
FIG. 3 is a graph showing how developing bias and density change
with respect to the number of print sheets in the first embodiment
of the image forming apparatus of the present invention.
[0134] FIG. 3A shows how the developing bias for printing changes,
and FIG. 3B shows how the density changes.
[0135] In FIG. 3A, the solid line E indicates how the developing
bias changes in this embodiment, and the dotted line F indicates
how the developing bias changes in the conventional control.
[0136] The image density control is executed for 100 print sheets
(as indicated by X1 and X2 in the drawings).
[0137] In FIG. 3B, the solid line D indicates how the image density
changes when this embodiment is adopted, and the dotted line C
indicates how the density changes in the conventional control.
[0138] In the conventional density control, the print developing
bias is updated immediately after the execution of image density
control, so that the change in density before and after the control
is rather great, whereas in the bias control of this embodiment, no
abrupt change in density occurs.
[0139] As described above, in this embodiment, the image formation
condition is gradually increased or decreased from the first image
formation condition which has been selected toward the second image
formation condition calculated through image density control,
whereby it is possible to prevent an abrupt change in density.
Second Embodiment
[0140] Next, a second embodiment of the image forming apparatus of
the present invention will be described. In accordance with this
embodiment, there is provided an image forming apparatus in which a
gradual increase or decrease in image formation condition is
effected from a first image formation condition selected
immediately before the execution of image density control toward a
second image formation condition calculated through image density
control at a rate of change corresponding to the difference between
the first image formation condition and the second image formation
condition, whereby an abrupt change in density is prevented, and
the image density is prevented from being greatly deviated from a
proper density for a long period of time.
[0141] The general construction of this embodiment and the devices
with which it is equipped are the same as those of the prior-art
technique described with reference to FIGS. 8 and 9, so that a
detailed description thereof will be omitted, and FIGS. 8 and 9
will be referred to as appropriate.
[0142] In this embodiment also, the DC component of the developing
bias is used as the image formation condition to be changed for
image density control.
[0143] First, with reference to the flowchart of FIG. 4, the image
density control of this embodiment will be described in detail.
FIG. 4 is a flowchart illustrating an image forming operation
applicable to the second embodiment of the image forming apparatus
of the present invention.
[0144] First, when an image density control execution command is
input to the CPU 17 of the main body, an image density control
sequence is started.
[0145] Step 41, Step 42, and Step 43
[0146] Toner images for detection (toner patches) are formed on the
photosensitive drum 1, and the densities of the toner patches are
measured by the density sensor 9.
[0147] Further, from the result of the measurement of the toner
patch densities, the optimum developing DC voltage (optimum
developing bias) 0 which is a value in correspondence with the
second image formation condition is calculated.
[0148] The above method is similar to that of the first embodiment,
so that a detailed description thereof will be omitted.
[0149] Step 44, Step 45, and Step 46
[0150] A judgment is made as to whether image density control is to
be executed when the main body power source is ON (Step 44).
[0151] Similarly, a judgment is made as to whether image density
control is to be executed immediately after the replacement of the
cartridge (process cartridge A or the developing cartridge) (Step
45).
[0152] Further, a judgment is made as to whether image density
control is to be executed or not when a print command is received
when the apparatus has not been used for a long period of time (one
hour in this embodiment (Step 46).
[0153] In any case, it is desirable to use the optimum developing
bias value 0 calculated immediately after the density control, so
that the procedure advances to Step 47, where the optimum
developing bias 0 obtained through image density control is input
to the print developing bias value 1.
[0154] Step 48
[0155] When none of the conditions of Step 44, Step 45, and Step 46
applies, the image density control executed is that which is to be
conducted when printing has been performed on a predetermined
number of sheets (100 sheets in this embodiment).
[0156] In this case, variation is effected while gradually
increasing or decreasing the developing bias from immediately after
the image density control, calculating the rate of change a of the
developing bias used at this time.
[0157] In this embodiment, the rate of change of the developing
bias is calculated by the following equation:
Rate of change of developing bias=(optimum developing bias
0-developing bias 1 immediately before density control).div.K
(where K is a predetermined constant)
[0158] That is, in this calculation, the rate of change of the
developing bias is determined according to the difference between
the optimum developing bias 0 (the control value corresponding to
the second image formation condition) and the developing bias 1
immediately before density control (the control value corresponding
to the first image formation condition), so that regardless of the
magnitude of the difference, the developing bias used becomes equal
to the optimum developing bias when printing is performed on a
fixed number of sheets (represented by K in the above equation).
That is, when the developing bias for density control achieves the
level of K, the developing bias is changed to the optimum
developing bias.
[0159] However, even when the difference is large, it is possible
to prevent the image density from being greatly deviated from the
proper density for a long period of time.
[0160] It is desirable that the predetermined constant K be set to
an optimum value according to the characteristics of the
apparatus.
[0161] Briefly, when this constant K is set to a large value, the
fluctuation in density each time printing is performed is small.
Conversely, when it is set to a small value, the fluctuation in
density is large.
[0162] On the other hand, when the constant K is set to a large
value, the time it takes for the print developing bias 1 to
converge to the optimum developing bias 0 increases. Conversely,
when it is set to a small value, the convergence time decreases.
Taking this into consideration, the value of the predetermined
constant K is set to 25 in this embodiment.
[0163] The above image density control is conducted for each of the
colors, Y, M, C, and K to complete the image density control.
[0164] It goes without saying that the optimum developing bias
value 0, the print developing bias value 1, and the rate of change
of the developing bias are independently provided for each of the
colors (Y, M, C, and K) and are separately stored in the main body
memory.
[0165] The control of the developing bias at the time of printing
is the same as that in the first embodiment (FIG. 2).
[0166] Next, the changes in the developing bias and the density in
this embodiment will be described with reference to FIGS. 5A and
5B. FIGS. 5A and 5B are graphs showing how the developing bias and
the density change with respect to the number of print sheets in
the second embodiment of the image forming apparatus of the present
invention.
[0167] FIG. 5A illustrates how the developing bias for printing
changes, and FIG. 5B illustrates how the density changes.
[0168] In FIG. 5A, the solid line E indicates the change of the
developing bias in this embodiment, and the dotted line F indicates
the change of the developing bias in the conventional control.
[0169] The image density control is executed each time printing has
been performed on 100 sheets (as indicated by X0, X1, and X2 in the
drawings).
[0170] Further, in FIG. 5B, the solid line D indicates the change
of the image density when this embodiment is adopted, and the
dotted line C indicates the density change in the case of a
conventional control.
[0171] In the conventional density control, the print developing
bias is updated immediately after the execution of image density
control, so that the density change is very remarkable before and
after the control, whereas, when the bias control of this
embodiment is adopted, no abrupt change in density is caused.
[0172] Further, the rate of change of the developing bias is varied
according to the difference between the optimum developing bias and
the developing bias immediately before density control, so that,
even when the difference is large, it is possible to prevent the
image density from being greatly deviated from the proper density
for a long period of time (At point X1 in the drawing, the value of
the solid line D is not greatly deviated from the proper density A
for a long period of time).
[0173] As described above, in this embodiment, the image formation
condition is gradually increased or decreased from the first image
formation condition selected immediately before the execution of
image density control toward the second image formation condition
calculated through image density control at a rate of change in
correspondence with the difference between the first image
formation condition and the second image formation
condition,whereby an abrupt change in density is prevented, and it
is possible to prevent the image density from being greatly
deviated from the proper density for a long period of time.
Third Embodiment
[0174] Next, a third embodiment of the image forming apparatus of
the present invention will be described. In this embodiment, when
the difference between a first image formation condition selected
immediately before the execution of image density control and a
second image formation condition calculated through image density
control is smaller than a predetermined value, the second image
formation condition is used from immediately after the execution of
the image density control. Otherwise, the image formation condition
is gradually increased or decreased from the first image formation
condition selected immediately before the execution of image
density control toward the second image formation condition
calculated through image density control, whereby an abrupt change
in density is prevented, and it is possible to prevent the image
density from being greatly deviated from the proper density for a
long period of time.
[0175] In this embodiment also, the DC component of the developing
bias is used as the image formation condition to be varied so as to
control the image density.
[0176] Further, the general construction of the image forming
apparatus of the present invention and the device with which it is
equipped are the same as those of the conventional technique
described above with reference to FIGS. 8 and 9, so a detailed
description thereof will be omitted,and FIGS. 8 and 9 will be
referred to as appropriate.
[0177] First, with reference to the flowchart of FIG. 6, the image
density control of this embodiment will be described in detail.
FIG. 6 is a flowchart illustrating an image forming operation
applicable to the third embodiment of the image forming apparatus
of the present invention.
[0178] First, when an execution command for image density control
is input to the CPU 17 of the main body, an image density control
sequence is started.
[0179] Step 61, Step 62, and Step 63
[0180] Toner images for detection (toner patches) are formed on the
photosensitive drum 1, and the densities of the toner patches are
measured by the density sensor 9. Further, from the results of the
measurement of the toner patch densities, an optimum developing DC
voltage (optimum developing bias) 0 is calculated. The
above-described method is the same as that of the first embodiment,
so a detailed description thereof will be omitted.
[0181] Step 64, Step 65, and Step 66
[0182] Next, a judgment is made as to whether image density control
is to be executed when the main body power source is ON (Step
64).
[0183] Similarly, a judgment is made as to whether or not image
density control is to be executed immediately after the replacement
of the cartridge (process cartridge A or the development cartridge)
(Step 65).
[0184] Further, a judgment is made as to whether or not image
density control is to be executed when a print command is received
when the apparatus has not been used for a long period of time (one
hour in this embodiment) (Step 66).
[0185] In any case, it is desirable to use the value of the optimum
developing bias 0 calculated immediately after the density control,
so that the procedure advances to Step 69, where the optimum
developing bias 0 obtained through image density control is input
to the print developing bias value 1.
[0186] When none of the conditions of Step 64, Step 65, and Step 66
applies, the image density control is executed when printing has
been performed on a predetermined number of sheets (100 sheets in
this embodiment).
[0187] Step 67
[0188] Next, a judgment is made as to whether the difference
between the optimum developing bias 0 (control value corresponding
to the second image formation condition) calculated through image
density control and the developing bias 1 used immediately before
the density control (control value corresponding to the first image
formation condition) is smaller than a predetermined value .
[0189] When the difference is smaller than the predetermined value,
the difference in density before and after the control is not so
great even if the optimum developing bias 0 is used from
immediately after the density control.
[0190] Thus, in this case, by using the optimum developing bias
value 0 calculated immediately after the density control, control
is performed such that the proper density can be achieved
immediately (The procedure advances to Step 69).
[0191] It is desirable for the predetermined constant to be set to
an optimum value according to the characteristics of the apparatus.
Specifically, it is desirable for the value of to be set such that
the density fluctuation when the developing bias is varied by is
equal to the maximum value of the density fluctuation permissible
to the user. Taking the above into consideration, the predetermined
difference value is set to 20V in this embodiment.
[0192] Step 68
[0193] The rate of change of the developing bias used when varying
the developing bias while gradually increasing or decreasing it is
calculated. The method of calculating the rate of change of the
developing bias is the same as that in the second embodiment. Of
course, the value used when varying the developing bias while
gradually increasing or decreasing may be a predetermined value as
in the first embodiment described above.
[0194] The above image density control is performed for each of the
colors Y, M, C, and K to complete the image density control.
[0195] The developing bias control at the time of printing is the
same as that in the first embodiment described above (FIG. 2).
[0196] Next, the way the developing bias and the density change
will be described with reference to FIGS. 7A and 7B. FIGS. 7A and
7B are graphs showing how the developing bias and the density
change with respect to the number of print sheets in the third
embodiment of the image forming apparatus of the present
invention.
[0197] FIG. 7A shows the way the developing bias for printing
changes, and FIG. 7B shows the way the density changes
[0198] In FIG. 7A, the solid line E indicates the change in the
developing bias in this embodiment, and the dotted line F indicates
the change in the developing bias in the conventional control.
[0199] Image density control is executed each time printing has
been performed on 100 sheets (as indicated by points X0, X1, and X2
in the drawing).
[0200] In FIG. 7B, the solid line D indicates the change in the
image density when this embodiment is applied, and the dotted line
C indicates the change in the density in the conventional
control.
[0201] In the bias control of this embodiment, when the difference
between the optimum developing bias calculated through density
control and the developing bias immediately before the density
control is large, the developing bias is gradually varied from
after the execution of the density control, so that no abrupt
change in density is caused (at point X1 in the drawing).
[0202] Further, when the difference between the optimum developing
bias calculated through density control and developing bias
immediately before the density control is small, it is possible to
quickly achieve the optimum density by using the optimum developing
bias from immediately after the execution of the density
control.
[0203] In this case, there is no fear that the difference in
density will become too large before and after the execution of the
density control (point X2 in the drawing).
[0204] That is, by adopting this embodiment, it is possible to
perform control so as to bring the image density closer to the
proper density while preventing an extreme variation in
density.
[0205] As described above, in this embodiment, when the difference
between the first image formation condition selected immediately
before the execution of image density control and the second image
formation condition calculated through image density control is
smaller than a predetermined value, the second image formation
condition is used from immediately after the execution of the image
density control. Otherwise, the image formation condition is
gradually increased or decreased from the first image formation
condition selected immediately before the execution of the image
density control toward the second image formation condition
calculated through image density control, whereby an abrupt change
in density is prevented, and it is possible to prevent the image
density from being greatly deviated from the proper density for a
long period of time.
[0206] While in the above-described embodiments of the image
forming apparatus of the present invention only the developing bias
is used as the image formation condition for the image density
control, it goes without saying that it is also possible to use
other image formation conditions, such as charging condition or
exposure condition (exposure amount), or arbitrarily combine them
for control.
[0207] In a conventionally well-known method, an optimum image
formation condition is calculated for each print from the condition
of use of the photosensitive drum or the developing device, the use
environment of the apparatus detected by an environment sensor,
etc., and is varied.
[0208] The above method, in which image density control is executed
for each print, is different from the present invention.
[0209] As described above, the image formation condition is
gradually increased or decreased from the first image formation
condition selected immediately before the execution of image
density control toward the second image formation condition
calculated through image density control, whereby it is possible to
prevent an abrupt change in density.
[0210] Further, by gradually increasing or decreasing the image
formation condition at a rate of change in correspondence with the
difference between the first image formation condition and the
second image formation condition, an abrupt change in density is
prevented, and it is possible to prevent the image density from
being greatly deviated from the proper density for a long period of
time.
[0211] Further, when the difference between the first image
formation condition and the second image formation condition is
small, the second image formation condition is used from
immediately after the execution of the image density control,
whereby it is possible to quickly achieve the proper density.
[0212] The above-described embodiments of the present invention
should not be construed restrictively. All manner of modifications
are possible without departing from the scope of the present
invention.
* * * * *